From b7dc5a071ddf69c0350396b203cba32fe5bab510 Mon Sep 17 00:00:00 2001 From: "Dmitry V. Levin" Date: Sat, 16 Feb 2019 16:10:39 +0300 Subject: [PATCH] parisc: Fix ptrace syscall number modification Commit 910cd32e552e ("parisc: Fix and enable seccomp filter support") introduced a regression in ptrace-based syscall tampering: when tracer changes syscall number to -1, the kernel fails to initialize %r28 with -ENOSYS and subsequently fails to return the error code of the failed syscall to userspace. This erroneous behaviour could be observed with a simple strace syscall fault injection command which is expected to print something like this: $ strace -a0 -ewrite -einject=write:error=enospc echo hello write(1, "hello\n", 6) = -1 ENOSPC (No space left on device) (INJECTED) write(2, "echo: ", 6) = -1 ENOSPC (No space left on device) (INJECTED) write(2, "write error", 11) = -1 ENOSPC (No space left on device) (INJECTED) write(2, "\n", 1) = -1 ENOSPC (No space left on device) (INJECTED) +++ exited with 1 +++ After commit 910cd32e552ea09caa89cdbe328e468979b030dd it loops printing something like this instead: write(1, "hello\n", 6../strace: Failed to tamper with process 12345: unexpectedly got no error (return value 0, error 0) ) = 0 (INJECTED) This bug was found by strace test suite. Fixes: 910cd32e552e ("parisc: Fix and enable seccomp filter support") Cc: stable@vger.kernel.org # v4.5+ Signed-off-by: Dmitry V. Levin Tested-by: Helge Deller Signed-off-by: Helge Deller --- arch/parisc/kernel/ptrace.c | 29 +++++++++++++++++++++-------- 1 file changed, 21 insertions(+), 8 deletions(-) diff --git a/arch/parisc/kernel/ptrace.c b/arch/parisc/kernel/ptrace.c index 2582df1c529b..0964c236e3e5 100644 --- a/arch/parisc/kernel/ptrace.c +++ b/arch/parisc/kernel/ptrace.c @@ -308,15 +308,29 @@ long compat_arch_ptrace(struct task_struct *child, compat_long_t request, long do_syscall_trace_enter(struct pt_regs *regs) { - if (test_thread_flag(TIF_SYSCALL_TRACE) && - tracehook_report_syscall_entry(regs)) { + if (test_thread_flag(TIF_SYSCALL_TRACE)) { + int rc = tracehook_report_syscall_entry(regs); + /* - * Tracing decided this syscall should not happen or the - * debugger stored an invalid system call number. Skip - * the system call and the system call restart handling. + * As tracesys_next does not set %r28 to -ENOSYS + * when %r20 is set to -1, initialize it here. */ - regs->gr[20] = -1UL; - goto out; + regs->gr[28] = -ENOSYS; + + if (rc) { + /* + * A nonzero return code from + * tracehook_report_syscall_entry() tells us + * to prevent the syscall execution. Skip + * the syscall call and the syscall restart handling. + * + * Note that the tracer may also just change + * regs->gr[20] to an invalid syscall number, + * that is handled by tracesys_next. + */ + regs->gr[20] = -1UL; + return -1; + } } /* Do the secure computing check after ptrace. */ @@ -340,7 +354,6 @@ long do_syscall_trace_enter(struct pt_regs *regs) regs->gr[24] & 0xffffffff, regs->gr[23] & 0xffffffff); -out: /* * Sign extend the syscall number to 64bit since it may have been * modified by a compat ptrace call -- 2.39.5